An acousto-optic filter ("AOTF") is an electronically tunable optical bandpass filter. Electronically tunable optical filters have been constructed so that an incident light beam of a first polarization is diffracted by an acoustic wave in a birefringent crystal to shift from the first polarization to a second polarization of the light beam for a selected bandpass of optical frequencies. The center wavelength of the passband of this type of filter is electronically tunable by changing the frequency of the acoustic wave within the crystal. Bulk AOTF's fabricated in bulk crystals and using bulk acoustic waves and unguided optical beams have already found many important applications in laser and optics systems. Integrated AOTFs in which light is confined to a waveguide and which use surface acoustic waves are also expected to find important applications in laser and fiber optics systems, especially such as are used in modern telecommunications applications.
As shown in FIG. 1, an integrated AOTF is fabricated in an elongated crystalline substrate 11 such as lithium niobate (LiNbO.sub.3). An optical waveguide 13 is formed in an upper surface of the substrate, for example by indiffusion of titanium. A beam of light is coupled into the waveguide 13 through an input optical fiber 17. The light propagates through the waveguide and out through an output optical fiber 19. A surface acoustic wave is induced in the waveguide by an interdigitated transducer 21. The transducer is driven by an externally-generated electrical signal from a signal source 22. The frequency of the acoustic wave is determined by the frequency of the electrical signal.
The acoustic wave induces a diffraction grating in the waveguide. The grating couples the transverse electric (TE) and transverse magnetic (TM) polarization modes of the light, but only within a narrow band of optical wavelengths. Thus, within this narrow band all the light propagating in one polarization mode is converted to the orthogonal mode, whereas outside this band the polarization mode of the light is unaffected
A TE pass polarizer 23 adjacent the first extremity of the waveguide blocks any incoming light that is not in a first polarization mode. Thus, only light polarized in the first mode is admitted to the filter. As the light travels through the waveguide, the polarization mode of any of the light having a wavelength within the narrow band of optical wavelengths is converted to a second mode which is orthogonal to the first mode. The polarization of the rest of the light is unaffected. A TM pass polarizer 25 opposite the polarizer 23 blocks from the output any light that is not in the second polarization mode. Thus, only light having a polarization mode that has been converted while passing through the filter is allowed to exit the filter. No output destination is shown, but it will be understood that the output light is ultimately provided to a user or to an optical device of some type.
The AOTF passes light having a wavelength within the band determined by the acoustic wave and blocks other light. Thus the AOTF serves as a bandpass filter. The center frequency of the pass band can be tuned by changing the frequency of the electrical signal that drives the transducer. The filter can be converted into a "notch" filter by changing the polarizer 25 to the same type of polarizer as the polarizer 23.
As light passes through the waveguide and is diffracted by the acoustic wave, the frequency of the light is Doppler-shifted because the grating induced by the acoustic wave is in motion with respect to the waveguide. If desired, this Doppler shift can be canceled by passing the light through a second AOTF.
FIG. 2 shows a prior art attempt to fabricate a polarization independent AOTF on a single substrate. A first optical waveguide 23 and a second optical waveguide 24 are formed on the upper surface of the substrate. A light beam of arbitrary polarization is coupled into the first waveguide 23 and the second waveguide 24 through an input TE-TM splitter 20. The TE-TM splitter 20 separates the TE and TM components of the incoming beam of light. The TE component is coupled into the first waveguide 23 and the TM component is coupled into the second waveguide 24. A surface acoustic wave is induced in the waveguides 23, 24 by an interdigitated traducer 21.
Within the narrow band of the filter all light propagating in one polarization mode is converted to the orthogonal mode. The TE component of the first waveguide within the narrow band of the filter is converted to TM and the TM component of the second waveguide within the narrow band of the filter is converted to TE. An output TE-TM splitter 24 combines the TM components within the first waveguide and the TE components of the second waveguide. The combined signals are passed to the output of the AOTF.
The prior art tunable band-pass filter shown in FIG. 2 has very limited out-of-band rejection. The TE-TM splitters 20, 24 are very hard to manufacture on the same substrate as the waveguides 23, 24. As a result, the isolation between the TE component coupled into the first waveguide 23 and the TM component coupled into the second waveguide 24 is very poor. An ideal TE-TM splitter will not have significant coupling between splitter outputs. That is, the TE component output of the ideal TE-TM splitter will not have a significant TM component and the TM component output of the ideal TE-TM splitter will not have a significant TE component. The coupling between the splitter outputs significantly reduces the effectiveness of the out-of band rejection of the filter.
AOTF construction as has been described is limited because the input beam of light must be constrained to a single polarization in order to obtain useable filter performance. In some applications, it is desirable to be able to filter a beam of light without constraining the incident light to a single polarization. A single fiber may couple several optical signals each at a different wavelength and each having a unique polarity to a single AOTF. It would be useful to be able to tune the pass band of the AOTF to be centered on a single one of the optical signals. It would also be useful if the AOTF were able to filter each optical signal individually without requiring the optical signals to be of a particular polarization.